In sheet-fed offset printing machines, a printing plate is drawn on around a printing plate cylinder by means of two tension rails arranged in the elongated cylinder channel. One tension rail is assigned to the leading edge of the printing plate (the print start) and the other is assigned to the trailing edge of the printing plate (the print end). Once the tension rails are clamped into position, external mechanisms tension the printing plate to the desired level. For this drawing-on operation, the plate cylinder is first rotated into a position in which the tension rail assigned to the leading edge is easily accessible such that the printing plate's leading edge can be introduced into the tension rail. After the leading print edge has been clamped, the plate cylinder is slowly rotated until the printing plate is wrapped around the plate cylinder and the trailing edge of the printing plate can be easily introduced into the print-end tension rail. It is of great importance to the success of the printing plate loading process that the leading print-start edge of the printing plate is exactly aligned with the printing plate and that the printing plate has been drawn on with a uniform tension. Any non-uniformities in the drawing-on moment over the format width must be corrected by a costly readjustment process.
Systems for automatically changing a printing plate, in which a new printing plate is fed and drawn onto the plate cylinder automatically, are known from DE 3,940,795 C2, DE 3,940,796 C2 and EP 0,431,575 A2. These systems also have, in addition to storage and reception regions for the printing plates, transport apparatuses for introducing new printing plates onto the print cylinder. These plate changing systems utilize pressure rollers which can be positioned against the outer circumference of the plate cylinder or positioned away from the plate cylinder. Once the pressure roller is engaged to the plate cylinder, the trailing print edge of the printing plate may be introduced into the corresponding tension rail. In the disclosure of EP 0,431,575 A2 the pressure roller is arranged on the pivotable end of a printing-plate magazine which can be set against the plate cylinder and, therefore, by reason of the lever relations recognizable in this publication, this pressure roller cannot exert a high contact pressure drawing-on force onto the outer circumference of the plate cylinder. EP 0,431,575 A2 further requires a pressure roller merely for introducing the folded print end of the printing plate into the tension rail.
The pressure roller suggested in the two German Patent Specifications, DE 3,940,795 C2 and DE 3,940,796 C2, serves to advance the non-folded end of the printing plate onto an open tension rail to facilitate a clamping operation. However, as illustrated in FIG. 3 of both publications, in order for the pressure roller to press the printing plate against the plate cylinder, the plate cylinder has to be moved relative to the pressure roller. This operation situates the pressure roller so that the roller can only partially and incompletely press the printing plate against the plate cylinder. The tightening torque previously obtained on the printing plate during the forward rotation of the plate cylinder in the course of the drawing-on operation can consequently be reduced in this phase.